Modeling Surfzone/Inner-shelf Exchange

The exchange of tracer (contaminants, pathogens, sediment, or larvae) between the surfzone and the inner-shelf, important to Navy operations, is poorly understood. Breaking-wave driven surfzone processes are radically different from the wind, stratification, (non-breaking) wave, and rotation driven processes in the inner-shelf. In the surfzone, rip currents (both transient and bathymetrically controlled) eject water onto the inner-shelf. Yet, rip ejection events are poorly understood. In addition, over a few hours dye tracer and surfzone surface drifters are “retained” within 2× surfzone width. Inner-shelf exchange is driven by wind, tides, and internal waves, and of most recent focus, by surface gravity waves. In < 10 m depth, onshore wave-induced Stokes drift balanced against Eulerian cross-shelf flow at depth can dominate other subtidal exchange processes. Diurnal and semi-diurnal internal waves as well as higher frequency internal bores also perform significant cross-shelf tracer exchange. The time-scales of the processes in the surfzone (< 10 min) and inner-shelf (hours+) are quite different. The interaction of these processes governs this exchange, and depends on the waves, wind, tide, and stratification. To study exchange between the surfzone and inner-shelf, I propose to:

Develop a model appropriate for studying surfzone/inner-shelf exchange by coupling the wave-resolving surfzone model funwaveC and ROMS. funwaveC has been validated for surfzone waves, currents, drifter and tracer dispersion, and runup. Finite-crest length wave breaking is incorporated into the model and thus it resolves rip current forcing mechanisms. The wave-averaged ROMS model (appropriate for the inner-shelf and some surfzone applications) incorporates all the inner-shelf exchange mechanisms, including those that are wave-induced. Two potential coupling paths will be explored.

Test and calibrate the model with coupled surfzone & inner-shelf observations from the ONR funded HB06 & IB09 experiments led by the PI. Both experiments had significant surfzone and inner-shelf dye, temperature, wave, current, and turbulence observations with which to compare to the coupled surfzone-inner shelf model. The observations of dye, temperature, and current vertical structure from the outer surfzone to inner-shelf will provide a challenging test for the model.

Use the model to diagnose mechanisms of tracer exchange and determine their importance given different incident wave, wind, stratification, tide, and Coriolis conditions that would be appropriate for various regions in the world of interest to the Navy. This work will quantitatively improve out ability to make predictions of the fate of pollutants and contaminants, in addition to other tracers, in the nearshore region.